Comparative Assessment of Different Electrode Combinations for Phosphate Removal from Onsite Wastewater via Electrocoagulation

Abstract

Phosphorus (P) discharge from onsite wastewater treatment systems (OWTSs) poses a significant threat to water quality, contributing to eutrophication in nutrient-sensitive aquatic environments. In treated effluents, P predominantly exists as orthophosphate (PO₄³⁻), a highly bioavailable and reactive form that requires targeted removal. This study evaluates the performance of electrocoagulation (EC) as a polishing step for PO₄³⁻ removal from OWTS effluents using 12 anode/cathode combinations comprising aluminum (Al), iron (Fe), magnesium (Mg), and stainless steel (SS). Key operational parameters, including treatment time, mixing speed, current density, pH, and initial PO₄³⁻ concentration, were systematically investigated when synthetic denitrified effluent (20 mg P/L) was treated. Based on the performance, the four most effective electrode combinations (Al/Al, Al/Mg, Fe/Al, and Mg/Mg), along with a commercial benchmark (Fe/Fe), were further tested under extended hydraulic retention times (up to 48 h) in both synthetic and real (denitrified) wastewater. To date, none of the studies have systematically evaluated all possible anode/cathode combinations involving multiple electrode materials under uniform operational conditions. The Al/Al and Mg/Mg EC systems achieved rapid and high PO₄³⁻ removal efficiencies (>95%), while Mg-based systems demonstrated sustained performance over prolonged treatment durations, especially in real wastewater. Bimetallic pairs such as Al/Mg and Fe/Al exhibited synergistic effects through enhanced coagulant generation and pH stabilization. The results indicated that PO₄³⁻ removal efficiency was strongly influenced by electrode material selection, hydrodynamic conditions, and wastewater compositions, underscoring the need to design EC systems based on site-specific water quality conditions in OWTSs.

1. Introduction

Phosphorus (P) discharge from onsite wastewater treatment systems (OWTSs) poses a persistent threat to water quality, especially in nutrient-sensitive watersheds [1]. Excess P loading stimulates excessive algal growth in receiving waters, leading to eutrophication, hypoxic conditions, biodiversity loss, and ecosystem disruption [2,3,4]. This is particularly critical in decentralized systems where OWTSs serve small communities or individual households, and their cumulative nutrient outputs can contribute significantly to watershed-scale pollution [5,6,7].

To mitigate these impacts, environmental agencies have imposed increasingly stringent effluent discharge limits for P, especially in regions such as the Great Lakes, Lake Cham/plain, and Lake Erie watersheds [8,9,10,11]. Polishing units, typically the final treatment stage in OWTSs, are designed to meet such limits by removing residual P [12]. However, conventional polishing technologies such as sand filters [13,14], constructed wetlands [15], or soil infiltration units [16] often underperform in P removal, particularly under low-strength wastewater conditions. These systems lack consistency due to media saturation and microbial fouling and often rely on physical or biological mechanisms that are insufficient for consistently achieving low P effluent levels [17,18].

In wastewater, P exists in various physical and chemical forms, including particulate P (PP), organic P (OP), and dissolved reactive P (DRP) or orthophosphate. Among these, orthophosphate (PO₄³⁻) is the most bioavailable and dominant dissolved species, particularly in treated effluents from OWTSs [7,18,19]. In typical OWTS effluent, total P (TP) concentrations ranged from 6.2 to 33 mg P/L, with approximately 84% present as PO₄³⁻ [20,21]. Orthophosphate is responsible for triggering eutrophication [22] and is therefore the primary target in advanced chemical and electrochemical treatment technologies [23,24,25]. A clear understanding of the speciation of P is therefore essential, as treatment processes like electrocoagulation (EC) are specifically designed to remove the soluble fraction through adsorption, co-precipitation, or complexation mechanisms involving metal hydroxides [26,27].

Electrocoagulation has emerged as a promising alternative to conventional chemical dosing methods due to its ability to generate coagulant species in situ through the electrolytic dissolution of sacrificial anodes [28,29,30,31,32]. This in situ generation reduces the need for external chemicals and allows for better process control through tunable parameters like current density, pH, mixing intensity, and hydraulic retention time [33,34,35]. Moreover, EC is particularly attractive for OWTSs due to its operational simplicity, small footprint, and potential for automation [36,37].

Commonly used electrodes in EC systems include aluminum (Al), iron (Fe), and magnesium (Mg), all of which offer low cost, high availability, and well-documented efficiency in PO₄³⁻ removal. Al³⁺ and Fe³⁺/Fe²⁺ ions form stable PO₄³⁻ precipitates over a broad pH range, while Mg²⁺ facilitates struvite (MgNH₄PO₄·6H₂O) formation when ammonium (NH₄⁺) is present [38,39,40]. The solubility and precipitation kinetics of these compounds are strongly influenced by system pH, ionic strength, competing ions, and electrode configuration [26,27]. The selection of electrode material not only governs the dominant removal mechanism but also influences energy consumption, electrode degradation, and sludge production during treatment [41].

Studies have shown that bimetallic anode/cathode combinations (e.g., Al/Fe, Fe/Al, Mg/Al) can outperform monometallic systems through synergistic coagulant generation and broader effective pH ranges [42,43]. It is, therefore, hypothesized that these bimetallic systems provide optimal ion dissolution and co-precipitation efficiency, especially under the low-contaminant conditions characteristic of polishing units in OWTSs. However, most existing research focuses on municipal or industrial wastewater with higher contaminant loads, making it less applicable to OWTSs treating relatively clean effluents. Furthermore, the long-term performance, electrochemical efficiency, and durability of various electrode pairings under intermittent loading and extended hydraulic retention time (HRT) remain underexplored. Extended HRT is believed to improve P removal by enhancing contact time between reactive coagulant species and PO₄³⁻ ions, thereby facilitating more complete precipitation reactions and minimizing interference from competing solutes. Nevertheless, systematic investigations of prolonged HRT effects on PO₄³⁻ removal using EC under realistic OWTS operating conditions are currently lacking. Therefore, the second hypothesis guiding this work is that extended HRT improves P removal efficiency through sustained exposure to reactive surfaces, better flocculation dynamics, and longer sedimentation time for insoluble PO₄³⁻ complexes.

Although EC has been widely applied to synthetic wastewater for PO₄³⁻ removal, most studies have tested only a single electrode combination, either monometallic or bimetallic and for relatively short treatment durations. To address these knowledge gaps, this study evaluates 12 different anode/cathode combinations using synthetic wastewater designed to mimic low-strength OWTS effluent. Operational parameters such as treatment time, mixing speed, current density, pH, and initial PO₄³⁻ concentration are systematically varied to assess their effects on P removal performance. This work aims to identify optimal electrode pairings and develop a more comprehensive understanding of how operational controls and electrode selection affect PO₄³⁻ removal performance in polishing units. Ultimately, the findings provide valuable insights to support the development of more efficient and sustainable P management strategies for OWTSs.

2. Materials and Methods

2.1. Wastewater Matrices

Synthetic wastewater was prepared using pure analytical-grade reagents (>99.7%) to simulate effluent characteristics from OWTS final treatment units (denitrifying woodchip bioreactor), which typically exhibit low organic and nutrient loads. The P source was potassium dihydrogen phosphate (KH₂PO₄), while sodium nitrate (NaNO₃) was added to mimic the background ionic strength and nitrogen (N) species commonly found in treated effluent. Solutions were prepared in deionized water to achieve an initial PO₄³⁻ concentration of 20 mg P/L, unless otherwise specified. The pH of the solutions was adjusted to the desired value using either dilute hydrochloric acid (HCl, 1.0 N) or sodium hydroxide (NaOH, 1.0 M), and was measured using a calibrated benchtop pH meter (HQ4300, Hach, Loveland, CO, USA). Real wastewater samples were collected from the final discharge point (denitrified effluent) of OWTS located in the Wastewater Research and Innovation Facility (WRIF) located at Stony Brook, NY, USA. Collected samples were stored at 4 °C and were used within 48 h from collection to minimize changes in composition. The wastewater was brought to room temperature before use. These samples had an initial PO₄³⁻ concentration ranging between 5.5 and 8.3 mg P/L.

2.2. Experimental Setup and Reactor Configuration

The schematic of the experimental setup is shown in Figure 1. All experiments were performed in a lab-scale batch reactor with a working volume of 1 L purchased from YASA Environmental Technology Co., Ltd., Shanghai, China. The reactor consisted of a square Plexiglas tank equipped with parallel electrodes placed vertically. Based on the earlier findings, an interelectrode gap of 2 cm was maintained throughout the experiments in this study to ensure effective PO₄³⁻ removal [44,45]. A DC power supply (BPS1203, Diaiary, Mainland China) was used to apply a constant current during treatment. The reactor was stirred using a magnetic stirrer (Cimarec+™, Thermo Fisher, Waltham, MA, USA) at speeds ranging from 0 to 600 rpm, depending on the experimental conditions. A total of 12 electrode combinations were tested, using commercially available metal plates of Al, Fe, Mg, and stainless steel (SS). The electrode dimensions were 12 cm × 10 cm × 0.3 cm with an effective surface area of 75.6 cm². The anode/cathode combinations included Al/SS, Al/Fe, Al/Mg, Al/Al, Fe/SS, Fe/Fe, Fe/Mg, Fe/Al, Mg/SS, Mg/Al, Mg/Fe, and Mg/Mg. Electrodes were polished with sandpaper, rinsed with deionized water, and dried before each experiment to ensure consistent surface conditions.

2.3. Experimental Design and Procedure

A traditional one-factor-at-a-time (OFAT) approach was employed to investigate the effects of individual operational parameters on PO₄³⁻ removal efficiency. As an optimization approach, OFAT allows systematic evaluation of individual operational parameters on treatment efficiency, helping to identify optimal conditions for target pollutant removal [44]. This approach provides foundational insights into process behavior before employing more complex multivariate optimization techniques [45]. The variables examined included treatment time (15, 30, 45, 60, 75, 90, 105, 120, 135, and 150 min), current density (0.25, 0.50, 0.75, 1.00, 1.25 mA/cm²), initial PO₄³⁻ concentration (5, 10, 15, 20, and 25 mg P/L), mixing speed (0, 150, 300, 450, and 600 rpm), and pH (6, 7, 8, and 9). Each parameter was varied individually while keeping all other parameters constant to isolate its influence on treatment performance. All experiments were conducted at ambient temperature (22 ± 2 °C). For each run, 1 L of synthetic wastewater was placed in the reactor and treated using a specific electrode pair. Samples were withdrawn at predetermined time intervals, filtered through a 0.45 µm membrane, and analyzed immediately. The PO₄³⁻ removal efficiency was calculated using the equation (Equation (1)) below.

$$\mathrm{R}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l} \mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y} (\%)=\left({\displaystyle \frac{C_o-C_t}{C_o}}\right)\times 100$$

(1)

where C_o indicates the initial PO₄³⁻ concentration and C_t denotes the PO₄³⁻ concentration at time t.

Based on the performance results with synthetic wastewater, four best-performing electrode combinations (Al/Al, Al/Mg, Fe/Al, and Mg/Mg) along with Fe/Fe (representing a commercial benchmark) were selected for further testing with synthetic and real wastewater (denitrified effluent) under extended hydraulic retention times (till 48 h).

2.4. Analytical Methods

Phosphorus concentrations were measured as PO₄³⁻ using the Hach Method 8048. All measurements were performed in triplicate, and blanks were used for quality control. pH was measured before and after treatment using a pH meter calibrated with standard buffer solutions. Sludge characteristics and electrode surface changes were observed visually, though not quantitatively analyzed in this phase of the study.

3. Results and Discussion

3.1. Effect of Treatment Time on Phosphate Removal

The effect of treatment time on PO₄³⁻ removal was evaluated using 12 anode/cathode combinations under identical operating conditions (initial PO₄³⁻ concentration: 20 mg/L, pH: 7, current density: 1 mA/cm², mixing speed: 0 rpm, interelectrode gap: 2 cm) (Figure 2a–c). In general, increasing treatment time improved PO₄³⁻ removal efficiency for all electrode pairs, although the rate and extent varied considerably depending on the anode and cathode materials.

In the Al-based systems (Figure 2a), PO₄³⁻ removal followed a time-dependent trend where Al/SS achieved a removal of 87.1% within 75 min but plateaued early. Al/Fe reached 91.7% removal in 105 min, showing a moderate but slower rate. In contrast, both Al/Mg and Al/Al demonstrated continued PO₄³⁻ removal over extended durations, achieving 94.3% and 96.7%, respectively, at 150 min. The sustained rise in removal efficiency with time in Al/Al reflects the continuous release and hydrolysis of Al³⁺ ions to form amorphous Al(OH)₃, which aggregates over time to sweep and adsorb PO₄³⁻ species [46]. The comparable performance of Al/Mg suggests that limited and incidental Mg²⁺ ions may occur from the cathode via localized corrosion associated with the negative difference effect, which in turn contributed to early-stage floc formation [47,48], although Al/Al maintained a stronger long-term coagulation effect.

Fe-based combinations displayed more varied time-dependent behaviors (Figure 2b). While Fe/Al progressively removed PO₄³⁻ up to 92.9% over 150 min, Fe/Fe reached only 50.9% within the same duration, indicating inefficient floc development. Fe/Mg and Fe/SS showed limited removal (27.2% and 13.2%, respectively) despite extended operation, suggesting poor coagulant formation and limited reactivity of cathode materials. The significantly better performance of Fe/Al with time highlights the benefit of using Al as a cathode to supplement Fe-based coagulation, possibly through dual coagulant generation (Fe(OH)₃ and Al(OH)₃) over time [26,49]. The slow accumulation of Fe(OH)₃ in Fe/Fe [50], coupled with the absence of synergistic effects in Fe/Mg and Fe/SS, limited their time-based effectiveness [51].

The Mg-based systems showed rapid PO₄³⁻ removal during the treatment (Figure 2c). Both Mg/Mg and Mg/Al achieved over 94% removal by 75 min, indicating efficient early floc formation dominated by the generation of Mg(OH)₂. The similar but slightly lower final performance of Mg/Fe and Mg/SS (88.7% at 105 min) suggests that while magnesium hydroxide precipitation is the dominant mechanism, cathode material influences removal kinetics over time. The high initial removal followed by an early plateau in Mg-based systems implies that Mg(OH)₂ rapidly saturates available PO₄³⁻ sites, and extended treatment beyond 75–105 min offers diminishing returns [40].

To sum up, treatment time had a significant impact on PO₄³⁻ removal, with longer treatment durations enhancing P removal efficiencies across all tested combinations. While all systems benefited from longer treatment durations, the rate and extent of removal varied due to differences in coagulant generation, floc growth kinetics, and cathodic contributions. The highest removal rates were observed in the Al/Al and Mg/Mg systems, emphasizing the importance of effective coagulant generation and adsorptive OH⁻ formation. These findings highlight the critical interplay between electrode material selection and operational time in optimizing PO₄³⁻ removal from OWTSs. The subsequent sections investigated the influence of other key operational parameters, such as pH, current density, and mixing speed, on the process performance.

3.2. Effect of Mixing Speed on Phosphate Removal

The effect of mixing speed on PO₄³⁻ removal efficiency was evaluated for 12 different anode/cathode combinations using EC under the following conditions: initial P concentration of 20 mg/L, pH 7, current density 1 mA/cm², and interelectrode gap of 2 cm (Figure 3a–c).

Among the Al-based systems, Al/Fe exhibited the highest PO₄³⁻ removal efficiency at 0 rpm (91.7%), followed by Al/Mg (90.2%), Al/SS (87.1%), and Al/Al (83.9%) (Figure 3a). The superior performance at no mixing conditions suggests that Al³⁺ ions were sufficiently dispersed by natural convection, and floc breakage was minimized [52]. In the Al/Fe combination, synergistic effects between Al³⁺ and Fe²⁺ enhanced charge neutralization and PO₄³⁻ complexation [36]. Al/Mg also performed well due to the co-precipitation of phosphate with Mg²⁺ and Al³⁺, which is consistent with studies emphasizing the benefit of bi-metallic systems in EC [41,42,43]. However, Al/Al and Al/SS, lacking additional catalytic ion release from the cathode, exhibited slightly lower efficiency under stagnant conditions.

For Fe-based systems, Fe/Al achieved the highest removal efficiency at 0 rpm (92.9%), followed by Fe/Fe (59.02% at 450 rpm), Fe/Mg (45.9% at 450 rpm), and Fe/SS (21.2% at 450 rpm) (Figure 3b). The outstanding performance of Fe/Al under stagnant conditions is primarily due to the combined action of Fe²⁺/Fe³⁺ generated from the anode and Al³⁺ from the cathode due to minor passivation, enhancing co-precipitation and electrostatic attraction mechanisms [43]. The relatively low removal efficiencies of Fe/Fe, Fe/Mg, and Fe/SS at higher mixing speeds can be explained by the destabilization of formed flocs under turbulence and the lower intrinsic coagulation capacity of Mg²⁺ and SS compared to Al³⁺ [51]. Also, high mixing speed may disrupt floc formation before effective sedimentation. Furthermore, the Fe/SS combination showed the lowest PO₄³⁻ removal, likely due to the inert nature of the SS cathode and reduced availability of active coagulant species [53,54], leading to insufficient pollutant capture.

In the Mg-based systems (Figure 3c), Mg/Fe at 450 rpm achieved the highest PO₄³⁻ removal (96.5%), outperforming no-mixing systems. This suggests that the co-generation of Mg²⁺ and Fe²⁺ together with vigorous mixing facilitated rapid coagulant dispersion and PO₄³⁻ precipitation (as Mg₃(PO₄)₂ and FePO₄) [36,51,55]. On the other hand, Mg/Mg (95.3%) and Mg/Al (94.7%) also performed well at 0 rpm, indicating that Mg-based flocculation is less dependent on external agitation [54]. The Mg/SS system, even at 600 rpm, maintained a high removal efficiency of 92.2%, which reinforces that Mg(OH)₂ precipitation is robust under both stagnant and turbulent conditions.

In general, the performance trends indicate that bi-metallic combinations generally outperform mono-metallic systems, particularly under no or low mixing speed conditions, where the stability of electrochemically generated flocs is better preserved. The results further underscore that careful selection of electrode materials significantly impacts the EC efficiency, depending on the synergistic metal ion release and the operational mixing conditions.

3.3. Effect of Current Density on Phosphate Removal

Current density plays a pivotal role in determining the coagulant dosage, bubble generation rate, and energy consumption during EC. As shown in Figure 4a–c, the PO₄³⁻ removal efficiency varied significantly across the 12 electrode combinations with respect to applied current densities, operational time, and hydrodynamic conditions.

Among the Al-based systems (Figure 4a), the Al/Mg combination achieved the highest removal efficiency (98.2%) at 1.0 mA/cm², benefiting from the combined action of Al³⁺ and Mg²⁺ species that form highly effective OH⁻ flocs [41]. The Al/Al pair also reached 98.2% removal but required a higher current density (1.25 mA/cm²), indicating that the increased dissolution of Al was necessary to achieve the same removal efficiency [56]. The Al/Fe and Al/SS combinations showed lower removal efficiencies of 91.7% and 87.2%, respectively, with Al/SS showing the lowest due to the inert nature of the SS cathode and lack of contribution to PO₄³⁻ binding species [51,57].

For Fe-based systems (Figure 4b), the Fe/Al pair showed the highest efficiency (94.2%) at 1.25 mA/cm², suggesting a beneficial interaction between Fe³⁺ and Al³⁺ in forming co-precipitated OH⁻ complexes for PO₄³⁻ removal [36,56]. The Fe/Mg system achieved moderate removal (73.2%) at a lower current density (0.5 mA/cm²), likely limited by the reduced generation of active coagulant species [57]. The Fe/Fe and Fe/SS systems showed significantly lower removal rates (59.0% and 29.7%, respectively), underscoring the limited role of Fe²⁺ species in PO₄³⁻ removal and the non-contributory nature of SS electrodes under these conditions [52,53].

Magnesium-based anodes consistently yielded high PO₄³⁻ removal efficiencies (Figure 4c). The Mg/Mg system achieved 95.3% at 1 mA/cm², highlighting the strong precipitation of Mg capability via Mg(OH)² formation, which adsorbs PO₄³⁻ effectively [55]. The Mg/Al (94.7%) and Mg/Fe (94.3%) combinations also performed well at 1–1.25 mA/cm², indicating the added benefit of Al³⁺ or Fe³⁺ as secondary coagulants [36]. These systems benefit from bi-metallic floc synergy without requiring excessively high current. While the current density is adequate, the lack of a reactive cathode limits total coagulant production. The Mg/SS pair also achieved high removal (92.2%), again indicating the strong capacity of Mg to dominate the coagulation process, even when paired with non-reactive electrodes [58].

Overall, higher current densities generally enhanced PO₄³⁻ removal by increasing coagulant release and bubble production. However, excessive current in some Fe-based systems may have led to destabilization of flocs or energy inefficiency.

3.4. Effect of pH on Phosphate Removal

The influence of pH on PO₄³⁻ removal efficiency was systematically evaluated across 12 anode/cathode configurations. Figure 5a–c demonstrates that neutral to slightly acidic conditions (pH 6–7) generally yielded higher removal, though optimal pH varied with electrode material due to their distinct hydrolysis and precipitation behaviors.

As shown in Figure 5a, among Al-based systems, the Al/Mg combination achieved the highest removal efficiency (98.2%) at pH 7. This superior performance can be attributed to the synergistic contribution of Al³⁺ and Mg²⁺ ions forming bimetal hydroxide flocs (Al(OH)₃ and Mg(OH)₂), which enhanced PO₄³⁻ adsorption through charge neutralization and sweep flocculation mechanisms [40]. The neutral pH (7) favored Mg(OH)₂ precipitation, while Al(OH)₃ formed efficiently under these conditions (pH 5–9) [59,60]. Al/Al demonstrated similarly high removal (97.6%) at pH 6, where slightly acidic conditions promoted optimal Al(OH)₃ formation with high adsorption affinity for PO₄³⁻ species [61]. Al/Fe (91.7% at pH 7) also showed effective performance, benefiting from the simultaneous hydrolysis of Al and Fe to produce mixed OH⁻ flocs, while Al/SS (87.2% at pH 7) performed reasonably well despite the cathodic inert nature of stainless steel, as Al remained the primary coagulant source.

In Figure 5b, Fe-based systems displayed optimal performance at alkaline pH. Fe/Al achieved 95.6% removal at pH 9, which aligns with the solubility minimum of Fe(OH)₃ around this pH, facilitating extensive precipitation and effective PO₄³⁻ binding [62]. The Al cathode likely aided further floc formation and reduced anode passivation. In contrast, Fe/Mg (72.9% at pH 7) and Fe/SS (62.1% at pH 9) showed moderate efficiency, constrained by either reduced floc volume or limited coagulant synergy. The Fe/Fe pair demonstrated the lowest efficiency (59.0% at pH 7) among Fe-based systems despite being operated at a higher current density (1 mA/cm²), possibly due to less effective Fe(OH)₃ formation under neutral pH and lower mixing, which restricted hydroxide polymer growth and phosphate interaction [46].

As illustrated in Figure 5c, all Mg-anode systems performed optimally near neutral pH. Mg/Mg recorded a removal efficiency of 95.3% at pH 7, with Mg²⁺ ions rapidly hydrolyzing to form Mg(OH)₂ precipitates known for their strong PO₄³⁻ affinity [40]. The Mg/Al pair (94.7%) exhibited similarly high removal at the same pH, suggesting that Al(OH)₃ addition contributed to increased floc density and stability. Mg/Fe (94.3%) also performed well, with Fe(OH)₃ aiding coagulation under neutral pH, while Mg/SS (92.2%) achieved slightly lower removal, likely due to limited cathodic involvement. Notably, the consistently high efficiency across Mg-based systems at pH 7 underscores the dominant role of Mg(OH)₂ flocs in PO₄³⁻ removal under neutral conditions.

These findings demonstrate that pH critically governs the speciation and precipitation behavior of metal OH⁻, thereby influencing PO₄³⁻ removal in EC processes. The optimal P removal was observed at pH 6–7 for most Al and Mg systems and pH 9 for certain Fe systems, aligning with the amphoteric nature of Al(OH)₃ and Mg(OH)₂ against the more alkaline precipitation behavior of Fe(OH)₃ [40,41,46].

3.5. Effect of Initial Concentration on Phosphate Removal

The impact of initial PO₄³⁻ concentration on removal efficiency was assessed across 12 anode/cathode combinations, revealing that Al- and Mg-based anodes consistently outperformed Fe-based systems under optimized EC conditions (Figure 6a–c).

Among the Al-anode systems, the Al/Mg combination achieved the highest removal (98.2%) at 20 mg/L (Figure 6a), benefiting from the strong electrochemical synergy between Al³⁺ and Mg²⁺, which formed diverse and structurally robust flocs capable of effectively binding PO₄³⁻. The abundance of PO₄³⁻ enhances the adsorption driving force and reduces the likelihood of coagulant underutilization [63]. Similarly, the Al/Al system (97.6% at 20 mg/L) demonstrated near-complete removal due to the elevated coagulant yield from both electrodes and an extended treatment time (150 min) (Figure 6b), which provided sufficient opportunity for PO₄³⁻ floc interactions to proceed to completion. In contrast, the higher PO₄³⁻ removal efficiencies for Al/SS (94.1%) and Al/Fe (93.3%) systems were observed at a lower initial PO₄³⁻ concentration (5 mg/L) (Figure 6a). The reduced PO₄³⁻ removal at higher initial concentrations (10–20 mg/L) in Al/SS and Al/Fe systems is primarily due to limited coagulant availability and inefficient floc formation. In Al/SS, only the Al anode contributes Al³⁺, which becomes insufficient as PO₄³⁻ concentrations increase. In Al/Fe, incomplete oxidation of Fe²⁺ and possible interference between Al(OH)₃ and Fe(OH)₃ reduce floc efficiency and adsorption capacity. As a result, both combinations perform well at low PO₄³⁻ concentration but are less effective when the coagulant demand exceeds the supply.

In Fe-anode systems, PO₄³⁻ removal efficiency varied significantly with initial PO₄³⁻ concentration due to differences in coagulant availability and floc formation dynamics (Figure 6b). The Fe/Al system performed best at 10 mg/L (97.1%), where the PO₄³⁻ concentration was optimally balanced with coagulant production, and the combination of Fe- and Al-based flocs provided synergistic removal. Fe/SS showed the highest removal at low concentration (77.5% at 5 mg/L) due to limited coagulant generation from the Fe anode alone. Fe/Mg and Fe/Fe demonstrated better removal efficiencies (72.8% and 59.0%, respectively) at 20 mg/L in comparison to other concentrations, but still lower than the removal efficiencies for Fe/Al and Fe/SS. This is primarily due to insufficient oxidation of Fe²⁺ to Fe³⁺ and a mismatch between PO₄³⁻ load and effective coagulant formation, along with increased passivation over extended treatment durations [56].

At an initial PO₄³⁻ concentration of 20 mg/L, all Mg-anode combinations performed well due to the strong coagulant generation capacity of Mg and optimal stoichiometry between Mg²⁺ and PO₄³⁻ ions (Figure 6c). The Mg/Mg system achieved the highest removal (95.3%) owing to maximal Mg²⁺ release from both electrodes, leading to abundant Mg(OH)₂ floc formation ideally suited for high PO₄³⁻ concentrations. Mg/Al (94.7%) and Mg/Fe (94.3%) systems also performed comparably, benefitting from mixed floc formation that enhanced binding capacity through additional surface functionalities. The Mg/SS system, while still effective (92.2%), showed slightly lower performance due to the inert nature of the SS cathode, which limited floc-forming reactions. These systems generally showed reduced performance at lower PO₄³⁻ concentrations (e.g., 5 or 10 mg/L) due to coagulant underutilization, charge imbalance, or limited ionic interactions, which diminish floc formation and pollutant capture efficiency.

Overall, these results confirm that both the anode material and initial phosphorus concentration significantly influence EC performance. Higher initial concentrations favor coagulant–contaminant contact, while appropriate anode/cathode pairings ensure sufficient and reactive hydroxide species for efficient PO₄³⁻ removal via charge neutralization, adsorption, and sweep flocculation mechanisms.

3.6. Effect of the Extended Operational Period on Phosphate Removal

Based on prior experimental screening of 12 electrode configurations, four electrode pairs, Al/Al, Al/Mg, Fe/Al, and Mg/Mg, were selected for extended evaluation, along with Fe/Fe, which is commonly used in commercial EC systems. These five combinations were tested for PO₄³⁻ removal from both synthetic wastewater (10 mg P/L) and real denitrified effluent (5–8 mg P/L) under extended hydraulic retention times (HRTs) up to 48 h. The results highlight the influence of electrode material, wastewater composition, and reaction time on EC efficacy (Figure 7a,b).

The Al/Al system showed a marked increase in PO₄³⁻ removal efficiency from 85.8% at 1–2 h to 97.6% at 4 h in synthetic wastewater (Figure 7a). However, the efficiency decreased to 90.2% and 87.2% at 24 and 48 h, respectively (Figure 7a). This decline is likely due to redissolution or destabilization of Al-P flocs caused by pH shifts and the saturation of binding sites over time. The aging of Al(OH)₃ flocs and potential electrode passivation may further reduce active coagulant production [27,46]. In contrast, PO₄³⁻ removal in real wastewater remained stable, reaching 98.8% at 24–48 h (Figure 7b). The buffering capacity and organic matter present in real effluent likely helped stabilize the pH and floc structure, enhancing overall performance [51,52].

For the Al/Mg combination, synthetic wastewater treatment exhibited fluctuating removal efficiencies: from 79.5% at 1 h to a peak of 92.4% at 2 h, followed by a slight decline and eventual recovery to 94.7% at 48 h (Figure 7a). These variations resulted from the differential dissolution rates and solubilities of Al and Mg coagulants [64]. Mg(OH)₂ formation, in particular, is highly sensitive to pH and contributes to adsorption and precipitation processes. The observed decline at 3 h may be attributed to temporary saturation or a lag in coagulant formation. In real wastewater, this bi-metallic combination maintained high and stable removal (97.6–98.8%) (Figure 7b), likely supported by the presence of humic substances and bicarbonates that stabilize the electro-generated flocs and support additional adsorption pathways [41,65].

The Fe/Al pair achieved rapid and high removal in synthetic wastewater, maintaining 98.2% efficiency up to 3 h before slightly declining to 94.7% at 24–48 h (Figure 7a). This trend suggests initial dominance of Fe(OH)₃ floc formation followed by possible re-dissolution or competitive ion displacement over time [46]. In real wastewater, PO₄³⁻ removal remained stable at ~96.5% for the first 24 h and then increased to 98.2% at 48 h (Figure 7b), indicating continued floc development and enhanced stability provided by organic ligands and other constituents in the effluent.

The Mg/Mg system showed a steady and progressive increase in PO₄³⁻ removal from 69.0% at 1 h to 98.1% by 24–48 h in synthetic wastewater (Figure 7a). This behavior is attributed to the gradual release of Mg²⁺ ions and formation of amorphous Mg(OH)₂ over time [55]. The relatively slower kinetics of Mg dissolution delay the initial floc formation but ultimately allow for high-efficiency PO₄³⁻ removal at extended HRTs [64]. Similar trends were observed in real wastewater, where removal increased from 91.2% to 98.2% across the 48 h period (Figure 7b). Initial interference from organic matter and competing ions may slow the removal process, but extended exposure ensures that sufficient Mg(OH)₂ is available for PO₄³⁻ capture.

The Fe/Fe configuration, often used in commercial EC units, initially demonstrated poor PO₄³⁻ removal in synthetic wastewater (~67% up to 4 h), but efficiency increased significantly to 92.5% and 95.0% at 24 and 48 h, respectively (Figure 7a). The delayed performance is linked to the slow oxidation of Fe²⁺ to Fe³⁺ and the time-dependent formation of Fe(OH)₃ flocs under near-neutral pH conditions [65]. In real wastewater, PO₄³⁻ removal was consistently high (96.5–98.8%) (Figure 7b), suggesting that complexation with organic matter and buffering effects accelerated the formation and stabilization of Fe flocs [54].

The results presented above highlight the importance of selecting an appropriate electrode material with wastewater composition and desired HRT to achieve optimal PO₄³⁻ removal. Al-based systems exhibited rapid removal but were susceptible to floc destabilization under prolonged treatment, while Mg- and Fe-based systems required longer durations to reach peak performance due to their slower electrochemical kinetics. Real wastewater generally enhanced performance across all electrode combinations due to its buffering capacity, organic content, and presence of secondary ions such as NH₄⁺ and HCO₃⁻ [36,41]. These findings provide important insights for designing and optimizing EC-based polishing units for OWTSs.

4. Comparative Evaluation of Phosphate Removal Across Wastewater Matrices Using Electrocoagulation

Table 1. Summary of selected studies employing EC for PO₄³⁻ removal from different wastewater matrices.

Electrode (Anode/Cathode)	Wastewater	Initial PO43− Concentration (mg/L)	pH	Current Density (mA/cm2)	Mixing Speed (rpm)	Treatment Time (min)	PO43− Removal Efficiency (%)	Reference
Al/Al	Synthetic	50–500	4–7	0.25–1	150	4–80	54.6–98.1	[39]
Fe/Fe	Synthetic	100	7	0.5	150	20	93	[38]
Fe/Fe	Urine	26.1	8.2–9	40	150	20	98	[66]
Al/Al	Synthetic	25–400	3	8.3	–	5–40	27.7–100	[67]
Fe/Fe							14.3–100
Al/Al	Domestic	12.9	7.8	10		10	98	[68]
Al/Al	Synthetic	400	3	16.6	–	11.72	85.8	[69]
Al/Al	Synthetic	50	3	1	–	40	98.9	[70]
Fe/Fe							93.5
Al-Mg/SS	Synthetic	20–150	5	11	–	10–60	42–100	[71]
Al/Al	Synthetic	50	7	10	200	120	97.6	[72]
Al/Ti	Synthetic	52	4	2	–	50	~100	[36]
Fe/Ti						100
Al-Fe/Ti	Synthetic	52.1	4	2	–	80	99.9	[63]
Fe/Fe	Domestic	4.2–4.8	7	1	–	30	93.9	[73]
Al/Al	Synthetic	5–20	6	1.25	–	150	95.9–97.6	This study
Al/Mg			7	1	–		92.7–98.2
Fe/Al			9	1.25	–		95.6–97.1
Mg/Mg			7	1	–	90	62.9–97.6

summarizes selected studies employing EC for PO₄³⁻ removal from various wastewater matrices using different anode/cathode combinations. The table provides a comparative perspective by highlighting critical operational parameters such as initial PO₄³⁻ concentration, pH, current density, mixing speed, treatment time, and the corresponding removal efficiencies.

A wide range of electrode combinations has been reported in the literature, including Al/Al, Fe/Fe, Al/Mg, Fe/Al, and Fe/Ti. Aluminum-based electrodes (e.g., Al/Al) consistently demonstrated high PO₄³⁻ removal efficiencies (generally > 95%) under varying operating conditions, particularly at moderate current densities (1.0–2.0 mA/cm²) and neutral to acidic pH levels [39,67,68,69,72]. Iron-based electrodes (Fe/Fe) also yielded considerable PO₄³⁻ removal (>93%), as observed in urine and synthetic wastewater systems. However, performance varied more significantly with initial concentration and pH [66,70]. For instance, higher removal was achieved in more alkaline environments (pH 8.2–8.9) and lower concentrations [66]. In contrast, application of Fe-based EC system in domestic wastewater demonstrated a 93.9% PO₄³⁻ removal efficiency at a lower current density and treatment time, reflecting the complexity of the real wastewater matrix [73]. Mixed electrode pairs such as Al/Mg, Fe/Al, and Al/Mg-SS demonstrated similarly effective performance. The Al/Mg combination (tested in this study) achieved PO₄³⁻ removal efficiencies ranging from 92.7% to 98.2% at a relatively low current density (1.00 mA/cm²) and under a neutral pH condition. The Mg/Mg system also showed a broad removal efficiency range (62.9–97.6%) under similar conditions, indicating the viability of Mg-based systems in EC applications, particularly at lower initial PO₄³⁻ concentrations (5–20 mg/L). Overall, the comparative analysis underscores that PO₄³⁻ removal through EC is highly dependent on the synergy between electrode material, initial PO₄³⁻ concentration, pH, and current density.

5. Conclusions

This study demonstrated the feasibility and effectiveness of EC for PO₄³⁻ removal in polishing units of OWTSs. Twelve anode/cathode combinations were systematically evaluated under various operational conditions to identify optimal electrode configurations. Al/Al and Mg/Mg pairs achieved rapid and high phosphate removal efficiencies in synthetic wastewater, driven by the in situ formation of Al(OH)₃ and Mg(OH)₂ flocs, respectively. Bimetallic systems such as Al/Mg and Fe/Al exhibited enhanced performance due to synergistic coagulant generation and broader operational pH ranges. Extended HRT studies revealed that Mg- and Fe-based systems gradually improved removal efficiency, outperforming Al-based systems in longer treatments due to sustained coagulant release and greater floc stability. Moreover, real wastewater matrices improved overall removal efficiency across all tested configurations, highlighting the influence of natural buffering and organic components. To sum up, these findings underscore the importance of electrode material selection, operating conditions, and wastewater composition in designing EC systems for PO₄³⁻ removal in OWTSs as a polishing unit.